Studies are being conducted on the biosynthesis of throxine through an investigation of the action of purified thyroid peroxidase on the iodination and coupling of tyrosine-containing oligopeptides. Parallel studies are also being performed with lactoperoxidase and horseradish peroxidase for purposes of comparison. In addition, the action of intracellular tissue proteinases such as cathepsin D on protein and oligopeptide substrates is under investigation, in relation to the release of diiodotyrosine and thyroxine from thyroglobulin.